Power converters, e.g., DC-to-DC and AC-to-DC, typically have unique circuitry for a proper graceful start-up (soft start) and to develop correct operating voltage biases. This unique circuitry may require custom integrated circuits and/or proprietary designs which may increase the cost and delivery schedule of such power converters. FIG. 3 illustrates a schematic diagram of a prior art flyback converter. Transformer T1 is shown having a primary-side bias winding 302. This is used to bias the Primary-Side Controller device 301 via its VDD port. The voltage at VDD is cross-regulated to the output voltage Vo via transformer coupling. Therefore it is possible to regulate the voltage at Vo by a controller 301 monitoring the voltage at its VDD port. Typically regulating Vo using transforming coupling to the controller 301 is not accurate enough for most applications, therefore an additional feedback path is needed from the secondary-side of the flyback converter 300 to its primary side. Voltage reference 304 (U3) is a device that provides a precision reference (Vo is compared to that precision reference), a voltage error amplifier (with its compensation components for stability) and a driver for driving an optical isolation coupler (optocoupler) 303. The controller 301 also contains a precision reference and voltage amplifier, but these circuits are not utilized when the additional feedback path is included. The optocoupler 303 is driven linearly. Therefore the current transfer ratio (CTR) of the optocoupler 303 is an issue. CTR adds gain to the additional feedback path. This gain can vary from device to device, and a device's CTR can change as it ages.
The controller 301 is located on the primary-side of the flyback converter 300. The secondary-side of the flyback converter 300 is where the load (application) is coupled. Typically the application device (not shown) contains a microprocessor with its capability of programmability. The controller 301 is isolated from the benefits that programming can provide for more sophisticated flyback converter control techniques. The power MOSFET switch Q1 is an external device, resistor R6 is an external resistor that scales the voltage analogous to current through the MOSFET switch Q1 and is used by the controller 301 for current sensing.